centering text in align*

Question

I have a project, I must use align*, but I have problem with it superficiality.

    \begin{align*}
        \span {n \choose n_{1}} &&\times&& {n-n_{1} \choose n_{2}} &&\times&& \cdots &&\times&& {n_{k} \choose n_{k}}\\
        &= \frac{n!}{n_{1}!(n-n_{1})!} &&\times&& \frac{(n-n_{1})!}{n_{2}!((n-n_{1})!-n_{2})!} &&\times&& \cdots &&\times&& \frac{n_{k}!}{n_{k}!1!}\\
        &= \frac{n!}{n_{1}!n_{2}! ... n_{k}!} \span\span\span\span\span\span
    \end{align*}

for above code I get this output:

but I want some thing like:

I tried \centering but it doesn't work and I tried \mkern+/-xmu but it doesn't have functionality,if text size or font is changed, then the superficiality result would change too.

@egreg I learned from someone.thanks for the advice,I will consider that. — ryhn, Sep 08 '21 at 15:32

score 5 · Answer 1 · answered Sep 08 '21 at 06:25

You can set content is similarly-sized boxes; this is automated using \eqmathbox[<tag>][<align>]{<stuff>} where the default <align>ment is centred.

\documentclass{article}
\usepackage{amsmath,eqparbox}
% https://tex.stackexchange.com/a/34412/5764
\makeatletter
\NewDocumentCommand{\eqmathbox}{o O{c} m}{%
  \IfValueTF{#1}
    {\def\eqmathbox@##1##2{\eqmakebox[#1][#2]{$##1##2$}}}
    {\def\eqmathbox@##1##2{\eqmakebox{$##1##2$}}}
  \mathpalette\eqmathbox@{#3}
}
\makeatother
\begin{document}
\begin{alignat}{4}
  \eqmathbox[box1]{\binom{n}{n_1}} & {}\times{} & 
    \eqmathbox[box2]{\binom{n - n_1}{n_2}} & {}\times{} & \cdots & {}\times{} & 
    \eqmathbox[box3]{\binom{n_k}{n_k}} \
  = \eqmathbox[box1]{\frac{n!}{n_1! (n - n_1)!}} & {}\times{} & 
    \eqmathbox[box2]{\frac{(n - n_1)!}{n_2!((n - n_1)! - n_2)!}} & {}\times{} & \cdots & {}\times{} & 
    \eqmathbox[box3]{\frac{n_k!}{n_k!1!}} \
  = \eqmathbox[box1]{\frac{n!}{n_1! n_2! \dots n_k!}}
\end{alignat}
\end{document}

It is good to know something new from the morning. :-) +1 – Przemysław Scherwentke Sep 08 '21 at 06:29 — Przemysław Scherwentke, Sep 08 '21 at 06:29

Zarko · Answer 2 · 2021-09-08T08:42:08.970

With array:

\documentclass{article}
\begin{document}
[\setlength\arraycolsep{2pt}
\begin{array}{*{8}{c}}
    & {n \choose n_{1}} & \times & {n-n_{1} \choose n_{2}} & \times & \cdots &\times & {n_{k} \choose n_{k}}      \[2ex]
=   & \frac{n!}{n_{1}!(n-n_{1})!} & \times & \frac{(n-n_{1})!}{n_{2}!((n-n_{1})!-n_{2})!} & \times & \cdots & \times & \frac{n_{k}!}{n_{k}!1!}        \[2ex]
=   & \frac{n!}{n_{1}!n_{2}! ... n_{k}!}& & & & & 
    \end{array}
]
\end{document}

Edit: Considering @Mico suggestion to use \displaymath equation size:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
[\setlength\arraycolsep{1pt}
\begin{array}{*{8}{>{\displaystyle}c}}
    & \binom{n}{n_{1}} & \times & \binom{n-n_{1}}{n_{2}} & \times & \cdots &\times & \binom{n_{k}}{n_{k}}      \[3ex]
=   & \frac{n!}{n_{1}!(n-n_{1})!} & \times & \frac{(n-n_{1})!}{n_{2}!((n-n_{1})!-n_{2})!} & \times & \cdots & \times & \frac{n_{k}!}{n_{k}!1!}        \[3ex]
=   & \frac{n!}{n_{1}!n_{2}! ... n_{k}!}& & & & &
    \end{array}
]
\end{document}

You may want to use display math mode rather than inline math mode for the binomials and the \frac expressions. — Mico, Sep 08 '21 at 06:50
@Mico, I left math terms as were written by OP. I will add suggested form ASAP. — Zarko, Sep 08 '21 at 06:58

score 4 · Answer 3 · answered Sep 08 '21 at 06:44

You say you "must use align". However, I can see no reason for not using an array environment, with automatic display math mode and two custom column types. Oh, and do please use \binom{...}{...} rather than { ... \choose ...}.

\documentclass{article}
\usepackage{array}   % for '\newcolumntype' macro
\newcolumntype{C}{>{\displaystyle}c}  % automatic display-math mode
\newcolumntype{O}{>{{}}c<{{}}} % column type for math operators
\usepackage{amsmath} % for '\binom' macro
\begin{document}
[
\setlength\arraycolsep{0pt}
\begin{array}{ *{4}{OC} }
  & \binom{n}{n_{1}} &\times& \binom{n-n_{1}}{n_{2}} &\times& \cdots &\times& \binom{n_{k}}{n_{k}} \[15pt]
= & \frac{n!}{n_{1}!,(n-n_{1})!} &\times& \frac{(n-n_{1})!}{n_{2}!,((n-n_{1})!-n_{2})!} &\times& \cdots &\times& \frac{n_{k}!}{n_{k}!,1!} \[12pt]
= & \frac{n!}{n_{1}!,n_{2}! \dots n_{k}!} 
\end{array}
]
\end{document}

score 2 · Answer 4 · answered Sep 08 '21 at 08:37

With a single alignment point and also eqparbox:

\documentclass{article}
\usepackage{amsmath}
\usepackage{eqparbox} 
\newcommand{\eqmathbox}[2][M]{\eqmakebox[#1]{$\displaystyle#2$}}
\begin{document}
\begin{align}
     \eqmathbox[M1]{\binom {n}{n_1}}& \times \eqmathbox[M2]{\binom {n-n_1}{n_2}} \times \cdots \times \eqmathbox[M3]{\binom{n_k}{ n_k}}\[0.5ex]
    = \eqmathbox[M1]{\frac{n!}{n_1!(n-n_1)!}} & \times \eqmathbox[M2]{\frac{(n-n_1)!}{n_2!((n-n_1)!-n_2)!}} \times \cdots \times \eqmathbox[M3]{\frac{n_k!}{n_k!,1!}}\
   = \eqmathbox[M1]{\frac{n!}{n_1!,n_2! ... n_k!} }
\end{align}
\end{document}

score 1 · Answer 5 · answered Sep 08 '21 at 15:08

Here are my solutions, the latter being similar to, but distinct from, Mico’s.

\documentclass{article}
\usepackage{amsmath}
\usepackage{array,booktabs}
\begin{document}
I usually start from the assumption that readers can read,
pairing up the terms by themselves, so large white space 
around operation symbols can be avoided.
\begin{multline}
  \binom{n}{n_{1}} \times \binom{n-n_{1}}{n_{2}} \times \dots \times \binom{n_{k}}{n_{k}} \
\begin{aligned}
&= \frac{n!}{n_{1}!,(n-n_{1})!} \times \frac{(n-n_{1})!}{n_{2}!,((n-n_{1})-n_{2})!} 
   \times \cdots \times \frac{n_{k}!}{n_{k}!,1!}\
&= \frac{n!}{n_{1}!,n_{2}! \dots n_{k}!}
\end{aligned}
\end{multline}
However, you can center the corresponding terms, if you so prefer;
an accurate comparison will tell you which display to use.
\begin{equation}
\setlength{\arraycolsep}{0pt} % let TeX do the spacing job
\begin{array}{
  @{}
  >{{}}c<{{}} % operator or relation
  >{\displaystyle}c % term
  >{{}}c<{{}} % operator or relation
  >{\displaystyle}c % term
  >{{}}c<{{}} % operator or relation
  >{\displaystyle}c % term
  >{{}}c<{{}} % operator or relation
  >{\displaystyle}c % term
  @{}
}
 & \binom{n}{n_{1}} &\times& \binom{n-n_{1}}{n_{2}} 
    &\times& \dotsb &\times& \binom{n_{k}}{n_{k}} \
\addlinespace
=& \frac{n!}{n_{1}!,(n-n_{1})!} &\times& \frac{(n-n_{1})!}{n_{2}!,((n-n_{1})-n_{2})!} 
    &\times& \dotsb &\times& \frac{n_{k}!}{n_{k}!,1!} \
\addlinespace
=& \multicolumn{7}{>{\displaystyle}l}{\frac{n!}{n_{1}!,n_{2}! \dots n_{k}!}} 
\end{array}
\end{equation}
Some filler text at the end, with the repeated recommendation to use the first
display and not the second one. Oh, I fixed a wrong factorial removing it.
\end{document}

Please, read the commentary.

centering text in align*

5 Answers5